The present invention relates to a radio communication system having a random access channel for transmission of an access request by a secondary station to a primary station, and further relates to primary and secondary stations for use in the system and to a method of operating the system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that such techniques are equally applicable to use in other mobile radio systems.
In a radio communication system it is generally required to be able to exchange signalling messages between a Mobile Station (MS) and a Base Station (BS). Downlink signalling (from BS to MS) is usually realised by using a physical broadcast channel of the BS to address any MS in its coverage area. Since only one transmitter (the BS) uses this broadcast channel there is no access problem.
In contrast, uplink signalling (from MS to BS) requires more detailed considerations. If the MS already has an uplink channel assigned to it, for voice or data services, this signalling can be achieved by piggy-backing, in which the signalling messages are attached to data packets being sent from the MS to the BS. However, if there is no uplink channel assigned to the MS piggy-backing is not possible. In this case a fast uplink signalling mechanism may be available for the establishment, or re-establishment, of a new uplink channel.
In many systems, for example those operating to the Global System for Mobile communication (GSM) standard and in the proposed UMTS standard, fast uplink signalling is enabled by the provision of a random access channel using a slotted ALOHA or similar protocol. A MS wishing to request a particular service from, or transmit a short message to, its BS transmits an access request on the random access channel, which request includes information identifying the MS together with data relating to the requested service or message (this data often being referred to as payload data). If the BS receives the access request successfully, it transmits an acknowledgement to the MS. If any further negotiation is required to set up a requested service this is normally done on a new channel, identified in the acknowledgement.
The present invention is concerned with the implementation of a random access channel operating with a Time Division Multiple Access (TDMA) protocol in which transmissions are required to take place in time slots having a predetermined duration. A MS selects one such time slot in which to transmit its access request. A major consideration in the implementation of such a scheme is the need to ensure sufficient capacity for the anticipated traffic load. A particular problem in the implementation of an effective scheme is that there are a number of different uses for the random access channel, each requiring a different amount of payload data to be transmitted to the BS. If the length of the time slots is made sufficient to cater for all possible applications the capacity of the random access channel may be unacceptably reduced. On the other hand, if the length of the time slots is set to provide adequate channel capacity, initiation of services requiring larger amounts of payload data than can be accommodated in an access request becomes unnecessarily complex.
In a Code Division Multiple Access (CDMA) system like UMTS the capacity of the access packet is limited by the minimum spreading factor available. In addition, the spreading factor may impose a limit on maximum communication range. This means that the maximum capacity may be reduced in large cells (or at least for the case of a MS near the cell boundary).
An object of the present invention is to address the problem of maximising the capacity of a random access channel.
According to a first aspect of the present invention there is provided a radio communication system comprising a primary station and a plurality of secondary stations, the system having a random access channel for transmission of an access request by a secondary station to a primary station, which access request comprises an access packet having a predetermined maximum data capacity, wherein a secondary station requiring to transmit an access request including more payload data than can be accommodated in the format of a single access packet has means for dividing the data into a plurality of parts and for transmitting an access request consisting of a plurality of access packets, each packet including one part of the payload data and in that the primary station has means for determining that a received access request comprises a plurality of access packets and means for combining the payload data from the plurality of access packets.
According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a random access channel for transmission of an access request by a secondary station to the primary station, which access request comprises an access packet having a predetermined maximum data capacity, wherein means are provided for determining that a received access request comprises a plurality of access packets and for combining the payload data from the plurality of access packets.
According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a random access channel for transmission of an access request by the secondary station to a primary station, which access request comprises an access packet having a predetermined maximum data capacity, wherein the secondary station comprises means responsive to determining that the secondary station requires to transmit an access request including more payload data than can be accommodated in the format of a single access packet for dividing the data into a plurality of parts and for transmitting an access request consisting of a plurality of access packets, each packet including one part of the payload data.
According to a fourth aspect of the present invention there is provided a method of operating a radio communication system, the system comprising a primary station and a plurality of secondary stations and having a random access channel for transmission of an access request by a secondary station to a primary station, which access request comprises an access packet having a predetermined maximum data capacity, wherein a secondary station which requires to transmit an access request including more payload data than can be accommodated in the format of a single access packet divides the data into a plurality of parts and transmits an access request consisting of a plurality of access packets, each packet including one part of the payload data, and the primary station determines that a received access request comprises a plurality of access packets and combines the payload data from the plurality of access packets.